The quest for high-performance organic thin-film transistor (OTFT) gate dielectrics is a field of intense current interest. In addition to having excellent insulating properties, such materials must meet key requirements for transition of OTFT technologies to practicality: inexpensive solution/low-temperature fabrication, mechanical flexibility, and compatibility with diverse gate materials and organic semiconductors. The resulting OTFTs should function at low biases to minimize power consumption, hence the dielectric must exhibit large capacitance.
OTFT-based electronic devices performing simple operations/functions offer unique attractions compared to traditional inorganics, including flexibility, light-weight, and inexpensive large-area coverage and integration. Although performance may be reduced compared to Si-based circuits, the aforementioned characteristics make this new technology attractive for enabling diverse new applications. To achieve these goals, OTFT semiconductor and dielectric components should ideally be readily fabricable via high-throughput, atmospheric pressure, solution-processing methods such as spin-coating, casting, or printing. Considerable research effort has targeted the development of solution-processable organic semiconductors, with recent impressive progress. However, the resulting OTFTs are limited by high operating voltages, typically >>10 V, due to the intrinsic low carrier mobilities of the semiconductors and the low capacitances of the dielectrics (typically thick SiO2 and polymer films). One approach to reducing OTFT operating voltages is to employ relatively thick, conventional high-dielectric constant oxide films, however, the vapor-phase deposition processes typically employed (e.g., sputtering) risk damaging organic components and yielding poor mechanical properties on flexible substrates. Furthermore, ultra-thin inorganic insulators are typically electrically “leaky”, rendering them incompatible with low-mobility semiconductors. Alternatively, ultra-thin solution-deposited, self-assembled organic mono- and multilayer dielectrics show promise; however, a pathway to efficient integration into large-volume coating processes is less obvious.
Polymers have been considered as gate insulator materials for OTFTs, in view of their ready processability from solution. Polymeric dielectrics such as poly(4-vinylphenol) (PVP), poly(methyl methacrylate) (PMMA), and certain polyimides have been investigated as gate insulators, however only a limited number of organic semiconductors were investigated, with the resulting OTFTs operating only at relatively high voltages. This reflects the substantial insulator thicknesses (usually >>0.3 μm) required to reduce gate leakage currents to acceptable levels, thereby affording low capacitance metrics (typically <<20 nF cm−2). An innovative alternative recently reported grows polymeric insulator in situ on the gate surface, and offers, in principle, tunable thickness control. However, reported capacitances are again modest (˜3 nF cm−2) for low-voltage OTFT applications. The thinnest polymer dielectrics achieved to date (50-100 nm) for polymer-based top-gate OTFTs operate at relatively low voltages (˜10 V) with a triarylamine semiconductor. This represents a significant advance, however the polymerization/annealing temperatures are quite high (230-290° C.) and the reported device I-V saturation characteristics not ideal.